1) Double check to ensure that you have ample free storage space available on your iPhone. Open Settings ➡️ General ➡️ About ➡️ Available: Here you will see the available storage space on your iPhone. You should maintain at least 15GB or more free storage space for the iPhone’s efficient operation.
OR
2) You may have a poor battery condition which cannot provide sufficient power for peak performance. What is the Battery Health of your iPhone? Open Settings ➡️ Battery ➡️ Battery Health and let us know which percentage you see for Maximum Capacity. Also note what is written under Peak Performance. If your battery's Maximum Capacity shows a value of 80% or less, then it has gone through its lifecycle and therefore should be replaced, according to Apple.
3) If that does not help, then contact 👉 Official Apple Support OR you can always get your iPhone examined at the Apple Store 👉 Genius Bar Reservation and Apple Support Options or at an Authorized Service Center for further diagnosis and evaluation.
Axel F.